Shaft seals of the aforesaid type are frequently used particularly on turbomachines, which have a shaft which is guided out of the casing and enables the connection of a drive or a driven unit. It lies within the nature of the shaft seal that on account of the relative movement of the shaft surface in relation to the adjacent casing a one hundred percent leak-tightness cannot be achieved. Particularly in the case of toxic or explosive process fluids, which are to be kept away from the environment by means of the shaft seal, the leakages must be carefully drained off. Also, in the case of steam turbines or gas turbines, for example, the process fluid is prevented from escaping to the environment by means of such a shaft seal and the leakage of the shaft seal or the amount of suction has a direct influence upon the resulting thermal efficiency. Minimizing the leakages of a shaft seal is one of the most important tasks within the scope of the design of such machines.
In turbocompressors, so-called tandem gas seals frequently undertake the task of sealing the pressure chamber inside the casing in relation to the atmosphere. The tandem gas seals are contactless seals and are lubricated with dry filtered sealing fluid or sealing gas.
A conventional arrangement with a shaft seal of the aforesaid type is represented schematically in FIG. 1. A shaft S extends through a penetration PT of a casing C. In the interior of the casing C there is a process fluid PF under a sealing pressure PPF. The process fluid PF is increased to the sealing pressure PPF by means of a compressor CO. Outside the casing C there is air AM under an ambient pressure PAM. A gap G between the shaft S and the casing C in the region of the penetration PT is sealed by means of a shaft seal SHS. The shaft seal SHS comprises a plurality of sealing modules SM, inter alia two main seals, these being a first main seal MS1 and a second main seal MS2. The two main seals MS1, MS2 are designed as gas seals DG1, DG2 or dry gas seals DGS. Starting from the interior of the casing C, provision is first made for two labyrinth seals, these being a pressure-increasing labyrinth seal LS1 and a second labyrinth seal LS2, between which a pressure-increasing sealing fluid SFP is introduced. The pressure-increasing labyrinth seal LS1 and the pressure-increasing sealing fluid. SFP have the purpose of increasing the sealing pressure to an at least necessary pressure level and are required only if the existing pressure in the compressor is lower than the at least necessary pressure level. Between the first main seal MS1 and the second, outer labyrinth seal LS2, moreover, a sealing fluid is fed into the gap G. As a consequence of the feed of sealing fluid SF, a mass flow is created through the first main seal MS1 to the outside and a mass flow is created through the labyrinth seal LS2 in the direction of the compressor. This mass flow as a rule is relatively low and does not build up any appreciable differential pressure in the labyrinth seal LS2. The mass flow of the pressure-increasing fluid SFP is measured so that together with the mass flow of the sealing fluid SF which flows through the labyrinth seal LS2 it builds up a differential pressure in the pressure-increasing labyrinth seal LS1 which in addition to the pressure in the compressor corresponds to the at least required pressure level. This mass flow flows back into the interior of the casing C. A third labyrinth seal LS3 is provided between the first main seal MS1 and the second main seal MS2. Between the third labyrinth seal LS3 and the second main seal MS2, an intermediate sealing fluid ISF is introduced into the gap G. Whereas the sealing fluid SF is a process fluid PF, the intermediate sealing fluid ISF is either an inert fluid or inert gas or the ambient medium, in most cases being nitrogen.
Between the first main seal MS1 and the second main seal MS2, specifically inwards, of the third labyrinth seal LS3, the mixture of sealing fluid SF and intermediate sealing fluid ISF, or process fluid and inert fluid or ambient fluid, which accumulates there, is drained off to a subsequent process which is not shown in more detail. The process can also be a torch by means of which the mixture is burned off. Frequently located outwards of the second main seal MS2 is an additional tandem arrangement of a labyrinth seal consisting of two seals LS4, LS5, between which a separation fluid SPF is introduced. A mixture of separation fluid SPF and intermediate sealing fluid ISF, which flows in the outwards direction through the second main seal MS2 as leakage, is directed by means of a second drain EX2 to a process or likewise to a torch.
Beneath the sealing arrangement, FIG. 1 shows the pressure pattern over the axial direction, from which results the flow directions through the seals. The dry gas seals are not arbitrarily reversible with regard to throughflow. In this respect, under specific operating conditions an increased amount of pressure-increasing sealing fluid SFP has to be supplied.
The arrangement of the gas seal which is shown in FIG. 1 is also referred to as a tandem gas seal. In the type of construction of the tandem gas seal with or without a labyrinth seal between the two main seals, the intermediate sealing fluid is only required with the type of construction with a labyrinth seal. An intermediate sealing fluid is normally nitrogen from an external source. Both partial amounts of the sealing fluid SF between the main seal MS1 and the additional shaft seal LS2 and partial amounts of the intermediate sealing fluid ISF between the main seal MS2 and an adjacent additional shaft seal LS3 are fed to the first drain EX1, wherein the pressures, as in the pressure pattern which is shown beneath the schematic arrangement in FIG. 1, are selected in such a way that the greater part of the supplied fluid volume finds its way into the first drain EX1. A smaller portion of the intermediate sealing fluid finds its way through the second main seal MS2 into the second drain EX2. The additional shaft seals LS4 and LS5 with the supplied separation fluid SPF essentially serves for shielding the second main seal MS2 against contamination of the environment AM, which may be contaminated as a result of, for example, oil mist from an adjacent bearing. The separation fluid discharges partly into the environment AM and it is partly drained off in the second drain EX2. For the sealing module SM, consisting of the additional shaft seals LS4, LS5, carbon rings or other types of seal can also be used.
In the case of low sealing pressures, it happens that the sealing pressure, by means of the additional pressure-increasing sealing fluid SFP in the first pressure-increasing labyrinth LS1, has to be increased so that there is a pressure gradient towards the first drain EX1. This is especially because the first main seal MS1, which is designed as a gas seal, always requires a pressure gradient from the space which is to be sealed, or from the sealing pressure, in the outward direction so as not to be destroyed with the rotation of the shaft. The second main seal MS2 must also always be acted upon by a pressure gradient Δ pMS2 in order to ensure a correct function and monitoring. A correct monitoring of a sufficient pressure gradient across the second main seal MS2 also has an important safety aspect in the case of this type of seal because a defective first main seal MS1 is to be determined by pressure increase or quantity increase in the fluid drain EX1 only when the second main seal MS2 is perfectly operational at this point in time. This pressure gradient across the second main seal MS2 is to be added to the pressure gradient Δ pMS1 and has to be built up in a pressure-increasing labyrinth seal LS1 on the process side. Depending on how high the pressure differential Δ pLS1 across the pressure-increasing labyrinth seal LS1 has to be increased, considerable amounts of pressure-increasing sealing fluid SFP are required for this purpose. This again impairs the efficiency of the entire plant to a significant degree.